Cobalt-base and nickel-base superalloys are known.
For example, components made from nickel-base superalloys are known, in which a γ/γ′ dispersion-hardening mechanism impacts the high-temperature mechanical properties. Such materials can have good strength, corrosion resistance and oxidation resistance along with good creep properties at high temperatures. When materials of this type are used in gas turbines, for example, these properties can allow for the intake temperature of the gas turbines to be increased and efficiency of the gas turbine installation can be increased.
By contrast, many cobalt-base superalloys can be strengthened by carbide dispersions and/or solid solution strengthening as a result of the alloying of high-melting elements, and this is reflected in reduced high-temperature strength as compared with the γ/γ′ nickel-base superalloys. In addition, the ductility can be impaired by secondary carbide dispersions in the temperature range of approximately 650-927° C. Compared with nickel-base superalloys, however, cobalt-base superalloys can have improved hot corrosion resistance along with higher oxidation resistance and wear resistance.
Various cobalt-base cast alloys, such as MAR-M302, MA-M509 and X-40, are commercially available for turbine applications, and these alloys have a comparatively high chromium content and are partly alloyed with nickel. A nominal composition of these alloys is shown in Table 1 in % by weight.
TABLE 1Nominal composition of known commercially availablecobalt-base superalloysNiCrCoWTaTiMnSiCBZrM302—21.558109.0———0.850.0050.2M50910.023.55573.50.2——0.60—0.5X-4010.525.5545.5——0.750.750.50——
However, it would be desirable to improve mechanical properties, such as the creep strength of these cobalt-base superalloys.
Cobalt-base superalloys with a predominantly γ/γ′ microstructure have also recently become known, and these have improved high-temperature strength as compared with the commercially available cobalt-base superalloys mentioned above.
A known cobalt-base superalloy of this type consists of (in at. % by weight):                27.6 Ni,        12.9 Ti,        8.7 Cr,        0.8 Mo,        2.6 Al,        0.2 W and        47.2 Co.(D. H. Ping et al: Microstructural Evolution of a Newly Developed Strengthened Co-base Superalloy, Vacuum Nanoelectronics Conference, 2006 and the 50th International Field Emission Symposium., IVNC/IFES 2006, Technical Digest. 19th International Volume, Issue, July 2006, Pages 513-514).        
Relatively high chromium and nickel contents, and additionally also titanium, are present in this alloy. The microstructure of this alloy includes a known γ/γ′ structure having a hexagonal (Co,Ni)3Ti compound with plate-like morphology, in which case the latter can have an adverse effect on high-temperature properties. The use of alloys of this type is limited to temperatures below 800° C.
In addition, Co-AM-base γ/γ′ superalloys have also been disclosed (Akane Suzuki, Garret C. De Nolf, and Tresa M. Pollock: High Temperature Strength of Co-based γ/γ′-Superalloys, Mater. Res. Soc. Symp. Proc. Vol. 980, 2007, Materials Research Society). The alloys investigated in this document each comprise 9 at. % Al and 9-11 at. % W, with 2 at. % Ta or 2 at. % Re optionally being added. This document discloses that the addition of Ta to a ternary Co—Al—W alloy can stabilize the γ′ phase, and the ternary system (i.e. without Ta) can have approximately cuboidal γ′ dispersions with an edge length of approximately 150 and 200 nm, whereas the microstructure of the alloy additionally containing 2 at. % Ta can have cuboidal γ′ dispersions with an edge length of approximately 400 nm.